gcc optimizations

[John Richard Moser]

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It looks like the redundancy analyzer in gcc is broken.  My first hint
was that after 6 hours of work I finally narrowed down some interesting
performance measurements using nbench[1]* to Partial Redundancy
Elimination; my second was popping into #gcc on OFTC and being told the
RA is "stupid" and doesn't work so hot on x86.

It turns out that -O2 turns -ftree-pre on by default, and -Os turns it
off; so certain operations were quite awesomely faster with -Os (and
others slower).  Fiddling with tons of optimizations I eventually
noticed the SAME hit -O2 takes in ONE optimization; I disabled this
during an -O2 compile of nbench and lo and behold the numbers looked a
lot better.

Details on the Wiki at [2].  Somebody should plug some real-world
application into this, like rendering a JPEG image, Web page, or Ogg
Vorbis file and measuring how long this takes in real time using both
'-O2' and '-O2 -fno-tree-pre'.  Some code seems slightly slower, while
most code seems significantly faster.


[1]http://www.tux.org/~mayer/linux/bmark.html
[2]http://wiki.laptop.org/go/User:Bluefoxicy/gcc_optimizations

*nbench is highly CPU intensive, it does no IO benchmarks.  The CPU will
get hot.  I compiled it on a tmpfs mounted on /tmp/x to avoid touching
flash.

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